426 Scott and Hill.—Slruchtre of Isoetes Hystrix. 
formation. The Monocotyledonous analogy was first clearly 
pointed out by Russow (1872, p. 158), though it had already 
been suggested, on less definite grounds, by v. Mohl (1845, 
p. 125) and Hofmeister (1852, p. 159). Later authors have 
usually referred to this analogy, which is of course of no value 
as an indication of affinity. 
Most investigators have regarded the stele of Isoetes as 
built up of leaf-traces, and as having no cauline portion. 
This is the view of Hofmeister, Russow, Sachs, De Bary, and 
among recent writers, of Farmer and Campbell. Farmer, 
however (1890, p. 40), allows that the distinction lies ‘rather 
in the mind of the investigator than in the actual object before 
him.’ It seems to us to be somewhat arbitrary to speak of a 
vascular cylinder as built up of leaf-traces when it is manifestly 
impossible to refer its constituent elements to the particular 
leaf-trace to which they belong. Isoetes appears to have a 
cauline stele just as any monostelic Lycopod has, only in the 
former the stele is much shorter than in most Lycopods, and 
the leaf-traces joining it more crowded. The stelar wood 
serves to join up the xylem of the leaf-traces, but does not 
belong to one trace more than to another, and in structure it 
differs obviously from that of the leaf-traces. We therefore 
prefer to adopt the view of Hegelmaier and Bruchmann 1 that 
the xylem-cylinder is cauline. The phloem is certainly cauline 
but hardly comes into the question, as the primary phloem 
can so rarely be identified. The view of the central cylinder 
as cauline of course applies to the adult stem only; in 
embryonic stages, according to all investigators, the construc¬ 
tion of the vascular system from the union of definite 
leaf-traces is indisputable. Similar differences between the 
embryonic and adult stem obtain in certain species of 
Lycopodium. 
Russow maintained that his ‘ phloem,’ i. e. the intracambial 
zone generally, was continuous with the phloem of the leaf- 
traces. Some other authors have disputed this, e. g. Mr. Wilson 
Smith in his recent paper (1900, p. 227). Our own observa- 
1 Hegelmaier, 1874, P* 5°5 5 Bruchmann, 1874, p. 570. 
